Fuel cell membrane hydration and fluid metering

ABSTRACT

A hydration system includes fuel cell fluid flow plate(s) and injection port(s). Each plate has flow channel(s) with respective inlet(s) for receiving respective portion(s) of a given stream of reactant fluid for a fuel cell. Each injection port injects a portion of liquid water directly into its respective flow channel. This serves to hydrate at least corresponding part(s) of a given membrane of the corresponding fuel cell(s). The hydration system may be augmented by a metering system including flow regulator(s). Each flow regulator meters an injecting at inlet(s) of each plate of respective portions of liquid into respective portion(s) of a given stream of fluid by corresponding injection port(s).

This application is a division of U.S. patent application Ser. No.08/899,262, filed Jul. 23, 1997, now U.S. Pat. No. 5,998,054.

STATEMENT OF GOVERNMENT RIGHTS

The Government of the United States of America has rights in thisinvention pursuant to Contract No. DE-AC02-94CE50389 awarded by the U.S.Department of Energy.

TECHNICAL FIELD

This invention relates, generally, to fuel cell assemblies and, moreparticularly, to membrane hydration and fluid metering in fuel cells.

BACKGROUND ART

A Proton Exchange Membrane (“PEM”) fuel cell converts the chemicalenergy of fuels such as hydrogen and oxidizers such as air/oxygendirectly into electrical energy. The PEM is a solid polymer electrolytethat permits the passage of protons (H⁺ ions) from the “anode” side of afuel cell to the “cathode” side of the fuel cell while preventingpassage therethrough of the hydrogen and air/oxygen gases. Some artisansconsider the acronym “PEM” to represent “Polymer Electrolyte Membrane.”The direction, from anode to cathode, of flow of protons serves as thebasis for labeling an “anode” side and a “cathode” side of every layerin the fuel cell, and in the fuel cell assembly or stack.

For instance, the PEM can be made using a polymer such as the materialmanufactured by E. I. Du Pont De Nemours Company and sold under thetrademark NAFION®. Further, an active electrolyte such as sulfonic acidgroups is included in this polymer. Also, the PEM is available as aproduct manufactured by W. L. Gore & Associates (Elkton, Md.) and soldunder the trademark GORE-SELECT®. Moreover, a catalyst such as platinumwhich facilitates chemical reactions is applied to each side of the PEM.This unit is commonly referred to as a membrane electrode assembly(“MEA”). The MEA is available as a product manufactured by W. L. Gore &Associates and sold under the trade designation PRIMEA 5510-HS.

An individual fuel cell generally has multiple, transversely extendinglayers assembled in a longitudinal direction. In the fuel cell assemblyor stack, all layers that extend to the periphery of the fuel cells haveholes therethrough for alignment and formation of fluid manifolds.Further, gaskets seal these holes and cooperate with the longitudinalextents of the layers for completion of the fluid manifolds. As iswell-known in the art, some of the fluid manifolds distribute fuel(e.g., hydrogen) and oxidizer (e.g., air/oxygen) to, and remove unusedfuel and oxidizer as well as product water from, fluid flow plates ofeach fuel cell. Furthermore, other fluid manifolds circulate water forcooling.

As is well-known in the art, the PEM can work more effectively if it iswet. Conversely, once any area of the PEM dries out, the fuel cell doesnot generate any product water in that area because the electrochemicalreaction there stops. Undesirably, this drying out can progressivelymarch across the PEM until the fuel cell fails completely.

Traditionally, attempts have been made to introduce water into the PEMby raising the humidity of the incoming reactant gases. That is, thefuel and oxidizer gases are often humidified with water vapor beforeentering the fluid manifolds in order to convey water vapor forhumidification of the PEM of the fuel cell.

However, problems can result from the use of water vapor inhumidification of the reactant gases. For example, significantquantities of heat are required in order to saturate the reactant gasstream at a temperature close to the temperature of the fuel cell. Inparticular, one cannot just employ waste heat from a cell coolingcircuit, because the temperature will necessarily be lower than the celltemperature. Furthermore, temperature variations within the reactant gasmanifolds and fuel cell plate channels can undesirably lead tocondensation of the vapor and poor distribution of the reactant gas andvapor/water.

Moreover, vapor distribution is unpredictable. So, despite theintroduction of water vapor into the gas stream at the inlet of alongitudinal fluid manifold, drying out of the PEMs can still occur.These drying problems of the fuel cell assembly can become severe athigh power levels.

Deleterious effects can also result from turns in the flow path of astream which is a mixture of water droplets and reactant gas. After thestream goes around a given curve, separation of the water from thereactant gas occurs. Anytime the stream changes direction and/orvelocity, the various settling rates yield -separation. Therefore, bythe time the stream reaches the end of such a flow path, most of theliquid will have settled out. Similar problems and unpredictability canresult in any unconstrained flow of water mixed with reactant gas.

Naturally, fuel cells within the same assembly or stack can have varyingefficiencies. In particular, some fuel cells generate more heat thanothers. A fuel cell running hot will require more water in order tofunction. If a fuel cell assembly delivers inadequate moisture to agiven fuel cell, then the PEM of that fuel cell begins to dry out, whichcauses it to run hotter still since the remaining fuel cells in theassembly continue to force high current therethrough. When the PEM of afuel cell completely dries out, that fuel cell begins to dry out anyadjacent fuel cells. Accordingly, it is desirable to deliver water toall the fuel cells in the stack.

Additional problems stem from height variations in different areas of anindividual fuel cell and the fuel cell assembly. For example, one canconsider a fuel cell assembly that is angled and sloping upward from itsentry end of a longitudinal reactant fluid supply manifold. There, themere injection of water at the entry of the manifold into the fuel cellassembly would undesirably result in fuel cells on the low end receivingall water and no gas (“PEM flooding”), and fuel cells on the high endreceiving all gas and no water (“PEM starvation”).

In one prior art attempt to address some of the problems outlined above,a system is designed so various waters cool the fuel cells in a stackand hydrate their respective PEMs. A hydrogen gas stream is delivered toeach anode plate. Injection ports from a water line mix liquid waterinto a given hydrogen gas stream before it arrives at an anode plate.The number of injection ports determines the amount of water injectedinto the gas stream, which thereafter flows to the anode plate.Nevertheless, there is no guarantee that every flow channel of the anodeplate will obtain from the humidified gas stream adequate water forhydration of its part of the PEM. Furthermore, the possibility of unevendelivery of water among flow channels represents a potential waste inthe system. Such a design is disclosed in U.S. Pat. No. 4,769,297 toReiser et al. (entitled “Solid Polymer Electrolyte Fuel Cell Stack WaterManagement System,” issued Sep. 6, 1988, and assigned to InternationalFuel Cells Corporation).

Thus, a need exists for ensuring effective, efficient, and continuoussupply of water to all active areas of a membrane of an individual fuelcell. A further need exists for ensuring all active areas of eachmembrane in a working section of a fuel cell assembly effectively andefficiently receive adequate water.

SUMMARY OF THE INVENTION

Pursuant to the present invention, the shortcomings of the prior art areovercome and additional advantages provided through the provision ofinjection ports which hydrate a fuel cell membrane by directly injectingliquid water into reactant fluid at inlet(s) of fuel cell platechannel(s). Furthermore, a flow regulator at a metering area distributesliquid water essentially evenly to multiple flow channels uniformly overthe volume of the fuel cell assembly.

According to the present invention, a hydration system can include aflow field plate or fuel cell fluid flow plate and an injection port.The plate has a flow channel with an inlet for receiving a portion of astream of reactant fluid for a fuel cell. The injection port is in fluidcommunication with the flow channel. In particular, the injection portinjects a portion of liquid water directly into the flow channel inorder to mix the portion of liquid water with the portion of the stream.This serves to hydrate at least a part of a membrane of the fuel cell.Further, the injection port can inject the portion of liquid water intothe inlet of the flow channel. The fuel cell can be a PEM-type fuelcell.

The plate can have a plurality of flow channels with respective inletsfor receiving respective portions of the stream of the reactant fluid.In addition, a plurality of respective injection ports in fluidcommunication with these flow channels can inject respective portions ofthe liquid water directly thereinto for mixing with the respectiveportions of the stream. This serves to hydrate at least respective partsof the membrane of the fuel cell.

In another aspect of the invention, a hydration system can include afuel cell fluid flow plate and first and second injection ports. Theplate has first and second flow channels with respective first andsecond inlets for receiving respective first and second portions of astream-of reactant fluid for a fuel cell. Moreover, the first and secondinjection ports are positioned at the respective first and second inletsfor injecting respective first and second portions of liquid water intothe respective first and second portions of the stream. This serves tohydrate at least respective first and second parts of a membrane of thefuel cell.

The first and second injection ports can inject the respective first andsecond portions of the liquid water directly into the respective firstand second flow channels.

In another embodiment of the present invention, a metering systemincludes a fuel cell fluid flow plate, an injection port, and a flowregulator. The plate has a flow channel with an inlet for receiving aportion of a stream of fluid for a fuel cell. The injection port ispositioned at the inlet for injecting a portion of liquid into theportion of the stream. The flow regulator meters the injecting of theportion of the liquid into the portion of the stream. The flow regulatorcan employ orifice metering. In addition, the flow regulator can includea porous block.

The plate can have a plurality of flow channels with respective inletsfor receiving respective portions of the stream of the fluid. Further, aplurality of respective injection ports positioned at these inlets caninject respective portions of the liquid into the respective portions ofthe stream. Moreover, the flow regulator can also serve to meter therespective portions of the liquid to be substantially equal in amount.The flow regulator can further serve to meter the injecting of therespective portions of the liquid into the respective portions of thestream. Where this liquid is liquid water, the metering can serve tohydrate at least respective parts of a membrane of the fuel cell.

In yet another aspect of the present invention, at least some of aplurality of fuel cell fluid flow plates can have one or more flowchannels with inlets thereon for receiving respective portions ofrespective streams of fluid. Plus, respective injection ports can bepositioned at these inlets for injecting respective portions of liquidinto the respective portions of the fluid. Finally, respective flowregulators can meter the injecting of the respective portions of theliquid into the respective portions of the fluid. A number of theseplates can form multiple fuel cells. Moreover, the respective flowregulators can distribute the liquid substantially uniformly among thefuel cells.

The invention further contemplates a method for providing metering ofliquid for a fuel cell. A plurality of portions of the liquid areinjected into respective portions of a stream of fluid received byrespective flow channels of the fuel cell. These injected portions ofthe liquid are metered to be substantially equal in amount.

Thus, the present invention advantageously provides direct injection andhydraulic metering of liquid water at the fuel cell plate inlet(s) ofeach fuel cell reactant gas stream in order to achieve adequate fuelcell membrane hydration and approximately equal flow in each channel ofeach plate, substantially uniformly in the assembly or stack.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention will be readily understood from thefollowing detailed description of preferred embodiments taken inconjunction with the accompanying drawings in which:

FIG. 1 is a front, sectional elevation view of one example of a fuelcell assembly incorporating and using the membrane hydration and fluidmetering system of the present invention;

FIG. 2 is a plan view of an interior face of one example of a fluid flowplate in a fuel cell of the assembly of FIG. 1;

FIG. 3 is a cutaway side view of multiple fluid flow plate(s) in stackedfuel cells as viewed along direction 3—3 of FIG. 2, depicting a flowregulator feature of the present invention; and

FIG. 4 is a cutaway side view of another example of a fuel cell fluidflow plate, in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In accordance with the principles of the present invention, a fuel cellassembly is provided in which injection ports directly injectapproximately equal amounts of liquid water into inlets of flow platechannels carrying reactant fluid for each fuel cell. Further, a flowregulator ensures substantially uniform metering of adequate water forhydration of all active areas of each membrane in the fuel cellassembly.

One example of a fuel cell assembly incorporating and using the novelfeatures of the present invention is depicted in FIG. 1 and described indetail herein.

In this exemplary embodiment, a fuel cell assembly 100 includes endplates 102 and 104, insulation layers 106 and 108, and current collectorplates 110 and 112, with a working section 114 therebetween, as will beunderstood by those skilled in the art. Further, a number of structuralmembers 116 join the end plates, as is well-known in the art.

Working section 114 includes a number of layers 118. The layersgenerally form fluid manifolds 150 for supplying fluids to and removingfluids from the working section, as will be understood by those skilledin the art. Preferably, a plurality of the layers form one or more(e.g., one hundred and eight) PEM-type fuel cells. The construction andutilization of such fuel cells is known in the art.

In a typical PEM-type fuel cell, the PEM is sandwiched between “anode”and “cathode” gas diffusion layers (not shown) that can be formed from aresilient and conductive material such as carbon fabric. The anode andcathode gas diffusion layers are sometimes referred to as “electrodes.”In particular, the anode and cathode gas diffusion layers serve aselectrochemical conductors between catalyzed sites of the PEM and thehydrogen and air/oxygen which flow in respective “anode” and “cathode”fluid flow plates.

By connecting an external load (not shown) between electrical contacts(not shown) of current collector plates 110 and 112, one can complete acircuit for use of current generated by the one or more PEM-type fuelcells.

One example of a layer 118 of working section 114 is depicted in FIG. 2as flow field plate or fuel cell fluid flow plate 120. The fluid flowplate has a face 122 with a number of parallel, serpentine flow channels124 that include respective inlets 126 and outlets 127. Further, thefluid flow plate has a number of peripheral holes 129 therethrough,which can cooperate in formation of the fluid manifolds of the fuel cellassembly.

Fluid flow plate 120 can be formed from a conductive material such asgraphite. For exemplary purposes, FIG. 2 illustrates face 122 as havingten flow channels 124. The flow channels are typically formed on thefade of the fluid flow plate by machining. As will be understood bythose skilled in the art, fluid flow plate 120 may be a bipolar,monopolar, anode cooler, or-cathode cooler plate. Further, face 122 isan anode side or cathode side of the fluid flow plate. Moreover, theflow channels carry an appropriate reactant fluid (e.g., hydrogen orair/oxygen). A typical fluid flow plate 120 might have dimensions of 9.5in. height, 8.0 in. width, and 0.06 in. thickness. Also, each flowchannel 124 on face 122 might have cross-sectional dimensions of 0.06in. width and 0.025 in. depth.

A flow regulator at metering area 130 for each fluid flow plate 120 isproximate inlets 126, as described below. The metering area can bepositioned at a selected corner of the fluid flow plate.

FIG. 3 depicts multiple instances of fluid flow plate 120 in fuel cellassembly 100. For exemplary purposes, FIG. 3 illustrates each fluid flowplate as having six inlets 126 of six corresponding flow channels 124 onits face 122.

A bridge or cover plate 128 extends along face 122 and across inlets126, defining one opening or injection port 131 for each inlet 126 inaddition to an input orifice 132. The injection ports provide fluidcommunication between the inlets and a transverse channel 134 for eachfluid flow plate 120. Preferably, the injection ports are ofsubstantially equal size.

For example, cover plate 128 can be formed from stainless steel foil. Inparticular, the cover plate can be shaped by laser cutting or throughphotoetching. Additionally, injection port 131 can be made circular witha diameter of 0.005 to 0.010 in., depending on such factors as desiredwater injection rates. For instance, a given injection port can beformed by laser cutting. Also, input orifice 132 can be made circularwith a diameter of 0.005 to 0.020 in., depending on such factors aswater injection rates and the number of injection ports downstreamtherefrom. Further, a given input orifice can be formed by lasercutting. Alternatively, a given input orifice can include multiple,relatively smaller input orifices which are sized and positioned toprovide any desired flow restriction.

Transverse channel 134 is bounded on its sides at metering area 130 by asection cut out of the remainder of the corresponding transverse layer118 in fuel cell assembly 100. Preferably, the remainder of this layerincludes gaskets 136 and membrane or PEM 138, as will be understood bythose skilled in the art. For instance, one can cut a slot out of thePEM for this metering area. Further, one can slice holes through the PEMfor formation of the fluid manifolds. Otherwise, the PEM is preferablycoextensive with fluid flow plate 120, with a gas diffusion layer (notshown) positioned between active areas of the PEM and the fluid flowplate.

Furthermore, transverse channel 134 has one transverse extent bounded bycover plate 128 and an opposite transverse extent bounded by an adjacentlayer 118. Input orifice 132 of the cover plate provides fluidcommunication between the transverse channel and an input channel 140.The input channel provides fluid communication with a fluid manifold,which preferably supplies hydration water thereinto.

The present invention preferably employs orifice metering in order toindividually and directly inject substantially equal amounts of liquidwater into each fluid flow plate inlet 126 in various faces 122throughout fuel cell assembly 100. For instance, through selection ofrelative sizes between input orifice 132 and injection ports 131, onecan hydraulically meter direct liquid water injection into the reactantfluid/gas streams supplied to the inlets of flow channels 124 from agiven fluid manifold 150.

Successful operation of a PEM-type fuel cell requires continuous andwidespread hydration of its PEM 138. Preferably, injection ports 131serve as atomizers that stream fine liquid water particles for mixinginto the reactant gas flow existing in each flow channel 124.

Optionally, one can position porous block 142 in transverse channel 134in order to enhance the metering of the liquid water into inlets 126.The porous block may act as a filter and/or restrictor for thedistribution of liquid water to flow channels 124. For instance, theporous block might be formed from a porous plastic material, a sinteredmetal material, or a tightly-woven fabric material.

By injecting water directly at each inlet 126 of flow channels 124,which together preferably provide an electrochemical interface for allactive portions of PEM 138 through a gas diffusion layer (not shown),one ensures that no section of the PEM in a given fuel cell ever driesout. This prevents the previous problem of dryness progressivelymarching across the PEM until the fuel cell fails. Indeed, the fuel cellgenerates its own product water for self-hydration when it does not dryout. Downstream of the inlet, any succeeding turns in the flow channeland consequent separation of water from the gas therein are notproblematic because the water must remain in that flow channel all theway until it exits out outlet 127 (FIG. 2) from the flow channel andinto a discharge fluid manifold. In particular, there is no opportunityfor the water to migrate to another portion of the assembly 100 beforethis exit.

That is, pressure carries the resultant mixture of liquid water and gasreactant through each flow channel 124 for wetting of the correspondingPEM 138 and exiting from outlet 127 (FIG. 2) and into a correspondingdischarge manifold for that reactant. So, PEM 138 will not have areasthat are either flooded or starved.

Where fluid flow plate 120 is a bipolar plate, an opposite face 122′(not shown) similarly can have a metering area 130′ positionedapproximately at inlets 126′ of flow channels 124′, as will beunderstood by those skilled in the art. For instance, a repetition ofthe machining pattern depicted on the face in FIG. 2 on the oppositeface of that same fluid flow plate desirably would provide a meteringarea at a consecutive corner of the plate, which would allow convenientconnection to another fluid manifold for supply of an appropriatereactant fluid.

At metering area 130, the direct injection of liquid water at inlet 126of each flow channel 124 in a given fuel cell carries to PEM 138 morewater, and thereby yields more output from the fuel cell, than ispossible using conventional humidification of reactant gas. By formingthe metering area at face 122 on each fluid flow plate that carriesreactant gas, one can advantageously distribute liquid watersubstantially uniformly along multiple flow channels of a given plate,as well as in multiple fluid flow plates throughout fuel cell assembly100. Desirably, one thereby obtains a fuel cell assembly whose outputbecomes much easier to maintain because of the assurance all the fuelcells receive all the water they can use.

In a fuel cell assembly having a relatively large number of fuel cells,the injection system must be desensitized from differences in injectionpressures in order to maintain substantially equal amounts of flowthrough injection ports 131. For instance, these differences ininjection pressures can result from changes in attitude of the fuel cellassembly. For example, if it is desired to limit the injection pressurevariation in metering area 130 to plus or minus ten percent, onepreferably sizes input orifice 132 so that the pressure drop thereacrossis ten times the pressure differential likely to result from changes inphysical attitude of the fuel cell assembly. That is, one preferablysizes the injection ports to provide differential pressure thereacrosssufficient to cause the pressure in transverse channel 134 to overwhelmphysical effects such as capillarity.

As depicted in FIG. 4, another example forms input channel 140″ on aface 122″ of fuel cell fluid flow plate 120″ opposite to face 122 onwhich flow channels 124″ are formed, in accordance with the presentinvention. Injection ports 131″ are then provided which pass through thebottom of the flow channels for fluid communication with the inputchannel.

As will be understood by those skilled in the art, one can relativelysize input orifice 132 and injection ports 131 in order to substantiallyremove the effects of any existing or anticipated transverse heightdifferential in an individual fuel cell. Similarly, one can size theinput orifices and injection ports on cover plate 128 for each one ofmultiple fluid flow plates 120 in fuel cell assembly 100 in order tosubstantially remove the effects of any existing or anticipatedlongitudinal height differential. By appropriately sizing the ports andorifice(s) in order to desensitize fuel cell performance from moderatevariations in fuel cell attitude or position, one can simplify fuel cellsystem control.

For example, one can decide to equalize liquid water flow among a numberof channels 124 within a selected percentage. Such a selectionconsequently determines approximately how much pressure drop one needsacross input orifice 132 and injection ports 131, taking into account areasonable amount of geometric distortion which may result from movementof the fuel cells.

For instance, one can evaluate the physical size and expectedapplication (e.g., whether vehicular or static) for a given fuel cellassembly 100. Next, one can generate test data by running the fuel cellassembly. Analysis of the test data will show what performanceimprovement will result from injecting a certain amount of liquid waterinto any particular flow channel 124. On that basis, one can decide howmuch additional water one wants to add to a given flow channel.

Numerous alternative embodiments of the present invention exist. Forexample, metering area 130 could easily be positioned upstream ordownstream of inlets 126. Thus, one or more injection ports 131 couldeasily inject liquid water for mixing with respective portions of thereactant fluid streams just upstream of inlets 126. Alternatively, oneor more injection ports 131 could easily inject liquid water into one ormore flow channels 124 at a position somewhat downstream of inlets 126.Moreover, any desired number of fluid flow plates 120 in a given fuelcell assembly 100 could easily include metering area 130 of the presentinvention. Also, metering area 130 could easily be employed in a givenfuel cell assembly 100 with another liquid besides, or in addition to,water. Of course, any injection port 131 and/or input orifice 132 canhave any desired shape and/or size. Additionally, working section 114could easily include cooler plates and/or fuel cells other than PEM-typefuel cells.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the following claims.

What is claimed is:
 1. A fuel cell fluid flow plate, comprising a firstexterior face portion including a flow channel with an inlet forreceiving a portion of a steam of fluid for a fuel cell; and a secondexterior face portion, said first and second exterior face portionsgenerally directed in opposite directions; said plate including anopening for injecting a portion of liquid into said portion of saidstream, said opening extending between said inlet and said secondexterior face portion, and said second exterior face portion comprisinga passage for conveying said portion of liquid to said opening.
 2. Theplate of claim 1, wherein said liquid comprises liquid water, andwherein said injecting serves to hydrate at least a part of a membraneof said fuel cell.
 3. The plate of claim 1 in combination with ametering system, wherein said opening comprises a flow regulator formetering said injecting of said portion of liquid into said portion ofsaid stream.
 4. The combined plate and system of claim 3, wherein saidflow regulator employs orifice metering.
 5. A fuel cell fluid flowplate, comprising: a first exterior face portion including a flowchannel with an inlet for receiving a portion of a stream of fluid for afuel cell; and a second exterior face portion, said first and secondexterior face portions generally directed in opposite directions; saidplate including an opening for injecting a portion of liquid into saidportion of said stream, said opening extending between said inlet andsaid second exterior face portion, and wherein said first exterior faceportion includes a plurality of flow channels with respective inlets forreceiving respective portions of said stream, said plate furtherincludes a plurality of respective openings for injecting respectiveportions of said liquid into said respective portions of said stream,said openings extending between said inlets and said second exteriorface portion.
 6. The plate of claim 5 in combination with a meteringsystem, wherein said openings comprise a flow regulator for meteringsaid injecting of said respective portions of said liquid into saidrespective portions of said stream.
 7. The combined plate and system ofclaim 6, wherein said flow regulator employs orifice metering.
 8. Thecombined plate and system of claim 6, wherein said flow regulator alsoserves to meter said respective portions of said liquid to besubstantially equal in amount.
 9. The combined plate and system of claim6, wherein said liquid comprises liquid water, and wherein said meteringof said injecting of said respective portions of said liquid waterserves to hydrate at least respective parts of a membrane of said fuelcell.